In oil wells, natural gas wells, and the like (hereinafter also collectively referred to as “oil wells”), steel pipes referred to as Oil Country Tubular Goods (OCTG) are used for extraction of underground resources. The steel pipes are sequentially connected to each other, and threaded joints are used for the connection.
Threaded joints for steel pipes are classified into two types: coupling-type joints and integral-type joints. A coupling-type threaded joint is constituted by a pair of tubular goods that are connected to each other, of which one is a steel pipe and the other is a coupling. In this case, the steel pipe includes male threaded portions formed on the outer peripheries at both ends thereof, and the coupling includes female threaded portions formed on the inner peripheries at both ends thereof. Thus, the steel pipe and the coupling are connected to each other. An integral-type threaded joint is constituted by a pair of steel pipes as tubular goods that are connected to each other, without a separated coupling being used. In this case, each steel pipe includes a male threaded portion formed on the outer periphery at one end thereof and a female threaded portion formed on the inner periphery at the other end thereof. Thus, the one steel pipe and the other steel pipe are connected to each other.
Also, threaded joints for steel pipes are sometimes classified into types such as a flush type, semi-flush type, and slim type based on the outside diameter of the joint portion. A slim type threaded joint is defined using, as a measure, a coupling in which the area of the critical cross section is substantially equal to that of the pipe body (hereinafter also referred to as a “standard coupling”). Specifically, a slim type threaded joint refers to one in which the maximum outside diameter of the joint portion is smaller than the outside diameter of the standard coupling. A flush type threaded joint and a semi-flush type threaded joint are defined using, as a measure, the pipe body. Specifically, a flush type threaded joint refers to one in which the maximum outside diameter of the joint portion is substantially equal to the outside diameter of the pipe body. A semi-flush type threaded joint refers to one in which the maximum outside diameter of the joint portion falls between that of a flush type threaded joint and that of a slim type threaded joint. In short, regarding the joint portion, flush type threaded joints have the smallest maximum outside diameter, semi-flush type threaded joints have the next smallest maximum outside diameter, and the slim type threaded joints have the largest outside diameter.
The term “critical cross section” as used herein refers to a cross section at an end of the engagement region of the threaded portion, i.e., at a position where the area of the cross section for bearing tensile loads is a minimum. The position of the critical cross section and the cross-sectional area thereof are factors in determining the tensile strength of the threaded joint. There are no definitions for clearly distinguishing among the flush type, semi-flush type, and slim type. However, as a general rule for threaded joints for steel pipes currently on the market, those in which the maximum outside diameter of the joint portion is up to about 104% of that of the pipe body are referred to as the flush type or the semi-flush type, and those in which the maximum outside diameter of the joint portion is up to about 108% of that of the pipe body are referred to as the slim type.
In general, the joint portion at the tubular end where a male threaded portion is disposed is referred to as a pin because it includes an element that is inserted into a female threaded portion. On the other hand, the joint portion at the tubular end where a female threaded portion is disposed is referred to as a box because it includes an element that receives a male threaded portion. Pins and boxes both have a tubular shape because they are constituted by end portions of tubular goods.
In service environments, threaded joints for steel pipes are subjected to high pressure from fluids (e.g., gas or liquid) present in the exterior and the interior. Hereinafter, pressure from the exterior is also referred to as external pressure and pressure from the interior is also referred to as internal pressure. Under these circumstances, threaded joints for steel pipes are required to exhibit high sealing performance against external pressure and internal pressure.
Threaded joints for steel pipes employ a tapered threaded portion constituted by a male threaded portion of the pin and a female threaded portion of the box. In threaded joints having a tapered threaded portion, the male threaded portion of the pin and the female threaded portion of the box engage in intimate contact with each other. Thus, a thread seal is formed. In addition, in order to supplement the sealing performance of the thread seal, an external seal may be provided and an internal seal may be provided, separately from the thread seal.
An external seal is constituted by a sealing surface formed on the inner periphery of the end region of the box and a sealing surface correspondingly formed on the outer periphery of the pin. When these sealing surfaces come into interference contact with each other at high contact pressure, the external seal is formed. The external seal primarily serves to prevent external fluids from accidentally invading the region of the thread seal so as to contribute to ensuring sealing performance against external pressure.
An internal seal is constituted by a sealing surface formed on the outer periphery of the end region of the pin and a sealing surface correspondingly formed on the inner periphery of the box. When these sealing surfaces come into interference contact with each other at high contact pressure, the internal seal is formed. The internal seal primarily serves to prevent internal fluids from accidentally invading the region of the thread seal so as to contribute to ensuring sealing performance against internal pressure.
Typically, in the region of the external seal, the wall thickness of the box is thinner than that of the pin. Thus, in the case where high internal pressure has been loaded to the threaded joint and the threaded joint as a whole has swollen radially, the region of the external seal in the box easily expands radially to plastically deform. In this case, the region of the external seal in the box remains radially expanded after removal of the internal pressure, and as a result, the contact pressure between the sealing surfaces of the external seal is significantly decreased. If a high external pressure is loaded to the threaded joint in this state, the sealing surfaces of the external seal easily become separated. When this occurs, the external fluid invades the region of the thread seal beyond the external seal and finally invades the interior of the threaded joint. This problem is referred to as an external pressure leak.
In the past, various proposals have been made on techniques to ensure the sealing performance of the external seal. For example, techniques of amplifying contact pressure at the external seal are disclosed in U.S. Pat. No. 7,506,900 (Patent Literature 1), European Patent Application Publication No. 2325435 (Patent Literature 2), International Publication No. WO2009/083523 (Patent Literature 3), International Publication No. WO2011/044690 (Patent Literature 4), United States Patent Application Publication No. 2010/181763 (Patent Literature 5), and United States Patent Application Publication No. 2008/265575 (Patent Literature 6).
Patent Literatures 1 and 2 disclose a technique in which a nose portion is provided in the end region of the box. The nose portion extends along the pipe axis from an end of the sealing surface of the external seal, adjacent the end, and does not contact the pin. The stiffness of the nose portion itself increases deformation resistance of the region of the external seal. As a result, radially outward plastic deformation is inhibited in the region of the external seal of the box, whereby a sufficient contact pressure between the sealing surfaces is ensured.
Patent Literatures 3 to 5 disclose a technique in which a shoulder surface is provided on the end of the box. The pin has a shoulder surface corresponding to the shoulder surface of the box. The shoulder surfaces of the box and the pin are brought into contact and pressed against each other by the screwing of the pin and serve as a stopper for restricting the screwing of the pin. In addition, in a made up state (also referred to as “fastened state”), the shoulder surfaces of the pin and the box serve to impart the so-called thread tightening axial force to the load flanks of the male threaded portion of the pin and the load flanks of the female threaded portion of the box. The shoulder surfaces of the box and the pin are inclined with respect to a plane perpendicular to the pipe axis toward the direction in which screwing of the pin advances and they come into pressure contact with each other in a hooked manner. Because the pressure contact between the shoulder surfaces is in a hooked manner, the region of the external seal of the box receives a reaction force that acts in a direction to cause it to shrink radially. As a result, radially outward plastic deformation is inhibited in the region of the external seal of the box, whereby a sufficient contact pressure between the sealing surfaces is ensured.
The technique of Patent Literature 6 employs both the technique of Patent Literatures 1 and 2, in which a nose portion is provided in the end region of the box, and the technique of Patent Literatures 3 to 5, in which a shoulder surface is provided on the end of the box so as to come into pressure contact with the pin in a hooked manner.